涑水杂岩的地球化学特征

GEOCHEMISTRY OF ARCHEAN SHUSHUI COMPLEX

侵入于涑水杂岩的变质基性岩脉具有2264±219Ma的Rb-Sr等时年龄。涑水杂岩的组成和主量、微量、稀土元素地球化学特征可与世界上太古代高级变质地体对比,反映早期陆壳受控于上地幔垂直对流的演化特征。

Along the northern slope of Mt. Zhongtiao occurs discontinuously a high-grade metamor-phosed and deformed complex called the Shushui complex. Based on its geochemistry, the complex could be subdivided into three different types: (1) tonalitic, trondjemitic and grano-dioritic gneisses, (2) supracrustal rocks consisting of amphibolite, marble and quartzite and (3) late granites. Of the complex, TTG gneisses are the most common rocks formed in Late Archaean as evidenced by Rb-Sr isotope data. Rare-earth data, together with major and trace elements data, are presented for most of the rock types making up the complex. Basic gneiss has a smooth REE pathern with high Ba, Rb and P2O5 but low K/Rb, Cr and Ni, similar to the basic gneiss of Lewisian granulite-facies gneisses. Intermediate gneiss has more fractionated REE distributions (CeN/YbN = 4 arid 13 for two samples) with positive or no Eu anomalies. The REE patterns in the TTG gneiss also show strong f ractionation, and have no Eu anomalies. The amphibolite is characterised by constant ∑REE and LREE enrichment. Late granites con-sist of calc-alkaline (σ<2.3) and alkaline rocks (σ > 3.3). They all show REE enrichment and have more strongly fractionated REE distributions (CeN/YbN = 12-18) and negative Eu anoma-lies.The geochemical data together with other lines of geological evidence suggest that (1) basic gneisses were produced by partial melting of mantle peridotite, followed by fractional crystallization, and (2) TTG gneisses produced by varying degree of melting of a mafic source. The most suitable tectonic environment to account for the generation of both types would be one similar to the underplate setting.